This invention relates generally to refrigeration and more specifically to cryogenic refrigeration systems.
Cryogenic refrigerators, also known generally as cryocoolers, are needed to create refrigeration for superconductors, power transformers, magnetic resonance imaging, cryo surgery, and other cryogenic applications. There exist several known ways of supplying refrigeration at cryogenic temperatures.
One such technique involves the use of pulse tube refrigerators. U.S. Pat. No. 4,953,366 discloses an acoustic cryocooler formed from a thermoacoustic driver driving a pulse tube refrigerator through a standing wave tube. Pulse tubes, generally, are well known to those skilled in the art. A conventional pulse tube refrigerator uses a compression space, a radiator, an accumulator and a pulse tube arranged in series so as to constitute a closed operating space. Within the system there is a certain amount of operating fluid, such as helium gas, the pressure of which varies during operation of the device during compression and decompression. This varying pressure leads to the establishment of a phase difference between the pressure vibration and the displacement vibration of the operating fluid, which in turn leads to heat absorption at a lower temperature terminal.
The pulse tube refrigerator disclosed in the ""366 patent includes a pulse tube, a first heat exchanger adjacent the pulse tube for inputting heat from a thermal load for cooling, and a second heat exchanger for removing heat transferred from the first heat exchanger across the pulse tube. Typically, the advantage to a pulse tube refrigerator is its lack of moving parts. Disadvantages include, however, relatively limited power and high specific power required to generate the (limited) refrigeration.
Additional known patents which cover variations of the pulse tube refrigerator include U.S. Pat. No. 5,275,002 to Inoue et al., U.S. Pat. No. 5,689,959 to Yatsuzuka et al., U.S. Pat. No. 5,711,156 to Matsui et al., U.S. Pat. No. 5,904,046 to Kawano, U.S. Pat. No. 5,966,942 to Mitchell, and U.S. Pat. No. 6,094,921 to Zhu et al.
A second known refrigeration device is commonly known as a Stirling machine and there are known variants related thereto. These too are generally well known to those skilled in the art. U.S. Pat. No. 4,143,520 to Zimmerman discloses, for example, a split Stirling machine. The split Stirling machine includes a displacer which fits loosely in a cylinder, with the cylinder connected to a piston chamber in which a piston is placed. The displacer interacts mechanically with the piston. When the displacer is in its lowest position, the piston is moved to its extreme compression position where it compresses the working fluid (typically helium gas) which thereby generates heat. As the displacer is then moved to the top of its cylinder, the warmed fluid in the displacer cylinder moves from the top of the cylinder to the bottom, with the bottom of the cylinder being at a lower temperature before the warmed fluid passes into this lower region of the displacement cylinder. After the warmed fluid moves into the lower region of the displacement cylinder, the piston is them moved to its extreme decompressed position, cooling the working fluid within the system. Then, when the displacer is moved back to its lowest position again, the cooled fluid is moved back to the top of the displacement cylinder, thereby completing the cycle.
Other patents known which purport to take advantage of the Stirling machine include U.S. Pat. No. 5,022,229 to Vitale, U.S. Pat. No. 5,477,686 to Minas, and U.S. Pat. No. 5,333,460 to Lewis et al. Generally, these devices create more refrigeration at a reasonable specific power, but have more moving parts as compared to the pulse tube refrigerators discussed above.
Some attempts have been made to join the pulse tube refrigerator technology with the Stirling cycle. U.S. Pat. No. 6,167,707 to Price et al. discloses a hybrid two stage expander having a first stage pulse tube expander. A common reciprocating compressor pneumatically drives both stages. The first stage Stirling expander purportedly provides high thermodynamic efficiency that removes a majority of the heat load from a gas within the cryocooler. The second stage pulse tube expander provides additional refrigeration capacity. The use of this system has the combined on drawbacks discussed above individually for each type of cryocooler.
Another group of cryocoolers has been developed specifically to cool superconductive magnets. These include baths in fluid cryogens, systems involving compression and expansion, cryogens with rare earth displacement materials used in regenerators, apparatuses to recondense vaporized helium, and hybrid systems. Several U.S. patents have issued in this area, including: U.S. Pat. Nos. 4,782,671; 4,926,646; 5,396,206; 5,442,928; 5,461,873; 5,485,730; 5,613,367; 5,623,240; 5,701,744; 5,782,095; and 5,848,532.
Still other known systems are based on magneto caloric effect, such as U.S. Pat. No. 4,599,866, or cyclically concentrating and diluting the amount of isotope 3He in a 3Hexe2x80x944He solution, such as that disclosed in U.S. Pat. No. 5,172,554.
Moreover, the prior art, although addressing the need for cryocooling, has not solved the problem of achieving a more efficient cryocooler which provides high levels of refrigeration at relatively low cost.
The present invention is a refrigeration method and apparatus for supplying refrigeration to a heat exchanger whereby refrigeration can be transferred from the heat exchanger to an external heat load such as the coil of a superconducting magnet or transformer.
Therefore, one aspect of the present invention is an apparatus for supplying refrigeration to an external heat source comprising, in combination, a first compressor for compressing a returning warmed cryogenic fluid stream to form a compressed stream; a heat exchanger for receiving and cooling the compressed stream by heat exchange with a returning stream used to form the returning warmed cryogenic fluid stream; means in the heat exchanger to separate the compressed stream into a major stream exiting the heat exchanger and a minor stream exiting the heat exchanger; an expander for expanding the major stream together with means to return an expanded major stream to the heat exchanger; means to expand the minor stream exiting the heat exchanger to further cool the minor stream; heat exchange means to use the minor stream to provide refrigeration to an external heat load; means to compress the minor stream after heat exchange with the external heat load and return the minor stream to the heat exchanger; and means to combine the major stream and the minor stream to form the returning warmed cryogenic fluid stream.
According to one preferred embodiment of the present invention, the heat exchange means used to provide refrigeration to the external heat load is a vacuum refrigerator which allows thermal contact between the working fluid of the refrigeration cycle and the external heat source. Alternatively, the working fluid in the refrigeration cycle can be the same fluid as that contained in a bath used to cool an external heat source. In this later embodiment, the cooling cycle is the same as described above but involves the reliquefaction of the vaporized coolant. The coolant, in this embodiment, absorbs heat as a liquid, is vaporized, is run through the cycle to be reliquefied, and is then returned to the cooling bath as a cold liquid.
Another aspect of the present invention is a method of supplying refrigeration to an external heat source comprising the steps of compressing a warmed return cryogenic fluid stream to form a compressed refrigerant stream; passing the compressed refrigerant stream into a heat exchanger for cooling by heat exchange with returning refrigerant; dividing the refrigerant stream into a major stream and a minor stream as it passes through the heat exchanger; taking the major stream from the heat exchanger and expanding the major stream to further cool the major stream prior to using the major stream as a heat exchange fluid for cooling the compressed refrigerant stream, taking the minor stream and expanding it to further cool the minor stream and using the minor stream to provide refrigeration to the heat load; and thereafter compressing the minor stream; and combining the compressed minor stream and the major stream at one of, before, during or after using the major stream and the minor stream in the heat exchanger to cool the compressed refrigerant stream, the combined major and minor streams after heat exchange forming the warmed return cryogenic fluid stream.